The present invention is directed to an improved method and apparatus for controlling operating temperatures within a furnace and in particular to a method and apparatus which compensates for thermal lags within the furnace and the interactions of the heat inputs of multiple heating units within a furnace.
The control system of the present invention is designed to be utilized in a furnace of a process for producing a product from molten material. While the control system is applicable to processes for producing many different products from various materials, for the purpose of illustrating the invention, the control system will be described in connection with a glass melting and refining process.
The furnace for such a process typically includes a melting and refining tank that is provided with at least one feeder for supplying raw batch material to the tank and a forehearth for distributing the molten glass to bushings, spinners, or other glass fiberizing means which are conventional in the art. Such furnaces typically include a series of burners, electrodes or other heat input means located along the length of the furnace to heat the glass as it passes through the tank and into the forehearth.
In order to properly melt and refine the glass in the furnace for a particular end use, a specific temperature profile for the glass should be established in the furnace. For example, in a furnace with a plurality of heating zones in the melting and refining tank, it may be desirable to have a glass temperature of about 2300.degree. F. at the batch feed end of the tank and a glass temperature of about 2500.degree. F. at the forehearth end of the tank with the glass temperature being held to certain desired temperatures in the regions intermediate the ends of the tank. Each of these intermediate temperatures may be different and in many cases are different from the temperatures in the other zones.
Attempts have been made to regulate the glass temperature within the furnace to obtain a desired temperature profile, such as the one described above, by means of a single regulator which is utilized to control the fuel-air supplies for all of the burners in the furnace. In one system, the control of the glass temperature is based on the regulation of the atmospheric temperature above the glass which is sensed by a single temperature transducer. It is not difficult to maintain the temperature adjacent the temperature transducer at its desired level. However, this system has two drawbacks. There is no one single glass or atmospheric temperature in the furnace. There are a multiplicity of different temperatures throughout the volume of the furnace in both the glass and the atmosphere over the glass. In addition, while measuring a single atmospheric temperature within the furnace is relatively easy and in certain cases controlling such an atmospheric temperature is not very difficult, the control system is not adapted to control temperatures in the glass being melted and refined, but only the hot exhaust gases above the glass. Consequently, while atmospheric temperature can serve as important guidelines in a furnace operation, it appears to be of greater importance to accurately measure the temperature within the glass in an attempt to regulate the glass temperatures throughout the furnace.
Another control system utilized in the prior art is one such as that disclosed in the patent to Griem Jr., U.S. Pat. No. 3,573,017, issued Mar. 30, 1971. This patent discloses a method and apparatus for melting and supplying heat softenable materials in a process wherein a crown temperature above the glass and glass temperatures are measured and fed to a furnace temperature controller which through a single regulator adjusts the fuel air supply to all of the burners within the furnace. The difficulty with this system is that the input from the different temperature sensers are utilized to adjust the heat input of all of the burners with no consideration being given to thermal lags within the furnace and the interaction between the heat input of the burners. The temperature in the throat could be at the desired level while the temperature at the rear of the furnace tank is too low. Yet to correct for the low temperature at the rear of the furnace the heat input of the burner at the throat is also increased thereby raising the temperature in that region above the desired level.
In other control systems, the furnace is divided into zones with each zone having an atmospheric temperature sensing means or glass temperature sensing means, burners and a fuel-air supply regulator for each set of burners. In this system, there is a temperature set point for each zone. The temperature sensed in each zone is fed independently into each individual regulator which attempts to maintain the heat input for its zone to maintain the predetermined set point temperature. This system generally performs very poorly. The heat input into one zone not only affects the regulated temperature in that zone but the regulated temperature in other zones as well. There is an interaction between zones which is not accounted for.
All of the systems discussed above have failed to adequately take into account at least two important aspects of the process. There is a thermal lag between a change in the heat input of a set of burners, electrode or other heating means and changes in the glass or atmospheric temperatures throughout the furnace. In fact, it can take several hours before an adjustment in heat input is reflected in a temperature change in some zones of the furnace. In addition, the heat input into one zone of a furnace affects the glass and atmospheric temperatures in other zones of the furnace. It is an object of the present invention to overcome the problems associated with the control systems of the prior art by accounting for both the thermal lags in the furnace and the interaction between various zones of the furnace due to the changes in the heat input into one or more of the individual zones.
In the present invention, an accurate control of a desired glass temperature profile within a furnace is obtained by dividing the furnace up into arbitrary zones or regions wherein each zone is provided with a means for heating the glass within the furnace, a means for adjusting the amount of heat input of the heating means and a means for sensing a temperature in each zone. To effect the control of the temperatures in the separate zones, the temperatures for each of the zones are sensed and recorded for a predetermined period and the heat input into the furnace for heating means of each zone is measured and recorded for the same period. If any of the sensed temperatures are not at the proper level, the heat input of the heating means, for at least one of the zones, is adjusted to cause the temperatures in each of the zones which are not at the proper level to approach the desired temperatures. This adjustment is made in response to the last sensed temperature for each of the zones in the aforesaid time period and at least some of the temperatures sensed and the heat inputs measured in each of the zones during the same time period so as to compensate for thermal lags within the furnace and the interaction between the heat inputs in the various zones.